Phage Mu repressor (Rep) establishes lysogeny by shutting down transposition functions needed for replication. Conformational changes of the C-terminal domain (CTD) play an important role in modulating DNA binding and repressor degradation by the Escherichia coil (E. coli) ClpXP protease. Degradation of the Rep induces Mu lytic development, in which Mu DNA replicates by transposing into multiple sites in the host chromosome. Our laboratory has defined the minimal sequence in the CTD required for ClpX recognition; nevertheless, the presence of this determinant alone is not sufficient to promote its degradation. Other factors such as the exposure and display of the determinant apparently play an essential role. The recognition determinant in Rep is not exposed in the native conformation; however, the virulent mutant repressor Vir induces movement of Rep CTD, exposing the determinant and promoting Rep degradation. We hypothesized that Rep is a sensor that responds to physiological signals by undergoing conformational changes to reduce its affinity for DNA and increase its susceptibility to ClpXP. In a natural environment, conditions such as stress, carbon deprivation and entry into stationary phase can generate Vir-like molecules, which may propagate the degradation signal to the unaltered repressor population. I will investigate the mechanism by which Rep's cryptic degradation signal becomes activated and examine the ability of ssrA-tagged repressor peptides to induce Rep conformational changes that lead to its degradation by ClpXP by pursuing the subsequent Specific Aims: 1) Evaluation of the mechanism by which conformational changes mediate bacteriophage Mu repressor degradation. 2) Comparison of CTD movements that modulate DNA binding and protease sensitivity; and 3) Assessment of the role of display and context of a CIpX recognition motif in regulating ClpXP degradation.